The invention relates to an ionizer and its use in an exhaust gas purification system for moisture-laden gases.
The ionizer is used for the charging of liquid and solid particles in process gases. In accordance therewith, reference is made in the specification to wet electric filters and dry electric filters.
DE 101 32 582 discloses an apparatus for the electrostatic cleaning of gases, specifically a wet electric filter apparatus. The apparatus is installed in the gas flow channel through which the gas to be cleaned is conducted into the apparatus. When the arrangement was reversed so that the gas flows from the bottom to the top it has been found that a water film is pushed from the bottom nozzle end to the upper nozzle end whereby the flow cross-section is narrowed. As a result, sparking occurs before the high voltage has reached such a value that sufficient ionization current can flow. This effect occurs particularly in connection with condensing and droplet-laden gases at speeds of 3 m/s of the gas flowing from the bottom upwardly through the nozzle. In addition, it has been observed that additionally the negatively charged center electrode pulls the water film floating at the edge practically weightlessly inwardly in the form of a torus which causes arcing.
U.S. Pat. No. 4,449,159 discloses a conical cylinder nozzle, a so-called venturi nozzle which is oriented horizontally and into which an electrode pin is inserted deep into the throat thereof. The electrode pin carries an ionization disc at the circumference of which the corona current flows to the anode by way of the gas. The thicker electrode pin serves at the same time as a focusing electrode.
In U.S. Pat. No. 4,247,307, the vertical spray wires of a wet tube electric filter are provided along the flow direction with serially arranged spray discs. The spray discs may have saw-tooth like structures at their circumference.
U.S. Pat. No. 5,254,155 furthermore discloses a central spray tube arranged in a hexagonal tube and provided with 6-cornered rings whose ends point in the direction of the corners of the hexagonal tube.
JP 200 11 98488 discloses an arrangement wherein alternatively discs and 8-toothed stars are disposed on a central spray wire.
The horizontal venturi nozzle of U.S. Pat. No. 4,449,159 is not suitable for droplet laden moist gas, since a water film is always carried along into the nozzle or, at lower gas speeds, water drips in the throat from the top onto the ionization disc and causes arcing. For a uniform current distribution over the circumference the disc must be accurately adjusted. This is however almost impossible in the hostile operation of the apparatus. Since the electrode pins must be inserted into the nozzle, assembly is expensive. The spray discs of U.S. Pat. No. 4,247,307 are intended to increase the ionization at their circumference whereas the ionization along the wire becomes smaller. By discs arranged in a row along the wire in the flow direction, the particle deposition is to be improved. The discs in connection with the increased ionization in that area however lead to an increased turbulence and renewed transverse mixing which does not improve the fine droplet extraction. If the disc is provided at its circumference with a saw-tooth structure providing for a large number of ionization points, the additional ionization effect is not essential because the equally charged zones located at short distances from one another are mutually repulsive. Furthermore, with respect to the gas flow direction, a serial arrangement of ionization zones is not effective since particles which are already in the vicinity of the wall of the deposition electrode are again entrained by the turbulence and the electrical wind so that, in the end, the probability of particle deposition is not improved.
In U.S. Pat. No. 5,254,155, in place of cylindrical tubes, hexagonal tubes are used together with six-corner rings arranged one after the other in the gas flow direction which cause the same problem.
For JP 200 11 98488, the same arguments apply—the subject matter differs only in that 8-corner stars are used alternatingly with discs.
Experiments have shown that the gas speed in the nozzle can be reduced to values below 3 m/s with a concurrent increase of the diameter and a reduction of the number of nozzles if, at the same time, the electrode is changed from a single point structure to a multi-point structure, for example, a seven point star electrode. If, for example, 1600 cubic meter per hour (Bm3/hr) of wet gas are conducted through 166 conical cylinder nozzles with a diameter of 24 mm an average nozzle gas speed of 5.9 m/sec and a maximum voltage at the electrode of 9 kV and about 30 μA ionization current per nozzle, corresponding to a total current of 5 mA are established. Per Bm3/h of gas only about 0.028 watts of ionizer power can be introduced in this way. Because of the effect of the raising water film as described above, arcing occurs from about 9 kV whereby the ionization is interrupted and the high-voltage power supply is highly strained.
It is the object of the present invention to prevent water films from raising along the nozzle walls.